Cropping and/or editing an UV map with multiple objects without joining them
A possible solution would be using the addon MultiEdit. You can edit multiple objects at once without joining them. 

Download: https://www.dropbox.com/s/9h7bqe7aza4b16p/MultiEdit_05.zip?dl=0

Blenderartist Thread: https://blenderartists.org/forum/showthread.php?339369-MultiEdit-version-5-Multiple-Objects-Editing

To install it you have to go to File -> User Preferences -> Install from File, then select the .zip and click Install from File. Then tick the checkbox to activate the addon. If you don't want to repeat this every time you start up Blender you can click Save User Settings. 




After installing it you can select all the objects you want to edit and click MultiEdit enter in the tool shelf (T). Then you can scale up your UV map. When you're finished you can click on MultiEdit exit to go out of edit mode. But be careful not to go back to object mode by pressing Tab or using the drop down menu at the bottom of the 3D View. This will join your objects.